# std::distance

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< cpp‎ | iterator

Defined in header `<iterator>` |  |
template< class InputIt > typename std::iterator_traits<InputIt>::difference_type distance( InputIt first, InputIt last ); |  | (constexpr since C++17)

Returns the number of hops from first to last.

If `InputIt` is not LegacyRandomAccessIterator, the behavior is undefined if last is not reachable from first.

If `InputIt` is LegacyRandomAccessIterator, the behavior is undefined if first and last are neither reachable from each other.

### Parameters

first | - | iterator pointing to the first element
last | - | iterator pointing to the end of the range
Type requirements
- `InputIt` must meet the requirements of LegacyInputIterator. The operation is more efficient if `InputIt` additionally meets the requirements of LegacyRandomAccessIterator.

### Return value

The number of increments needed to go from first to last.

The value may be negative if random-access iterators are used and first is reachable from last. | (since C++11)

### Complexity

Linear.

However, if `InputIt` additionally meets the requirements of LegacyRandomAccessIterator, complexity is constant.

### Possible implementation

See also the implementations in libstdc++ and libc++.

C++98 implementation via tag dispatch, with constexpr removed
    namespace detail
    {
        template<class It>
        constexpr // required since C++17
        typename std::iterator_traits<It>::difference_type
            do_distance(It first, It last, std::input_iterator_tag)
        {
            typename std::iterator_traits<It>::difference_type result = 0;
            while (first != last)
            {
                ++first;
                ++result;
            }
            return result;
        }

        template<class It>
        constexpr // required since C++17
        typename std::iterator_traits<It>::difference_type
            do_distance(It first, It last, std::random_access_iterator_tag)
        {
            return last - first;
        }
    } // namespace detail

    template<class It>
    constexpr // since C++17
    typename std::iterator_traits<It>::difference_type
        distance(It first, It last)
    {
        return detail::do_distance(first, last,
                                   typename std::iterator_traits<It>::iterator_category());
    }
C++17 implementation via if constexpr
    template<class It>
    constexpr typename std::iterator_traits<It>::difference_type
        distance(It first, It last)
    {
        using category = typename std::iterator_traits<It>::iterator_category;
        static_assert(std::is_base_of_v<std::input_iterator_tag, category>);

        if constexpr (std::is_base_of_v<std::random_access_iterator_tag, category>)
            return last - first;
        else
        {
            typename std::iterator_traits<It>::difference_type result = 0;
            while (first != last)
            {
                ++first;
                ++result;
            }
            return result;
        }
    }

### Example

Run this code

    #include <iostream>
    #include <iterator>
    #include <vector>

    int main()
    {
        std::vector<int> v{3, 1, 4};
        std::cout << "distance(first, last) = "
                  << std::distance(v.begin(), v.end()) << '\n'
                  << "distance(last, first) = "
                  << std::distance(v.end(), v.begin()) << '\n';
                  // the behavior is undefined (until LWG940)

        static constexpr auto il = {3, 1, 4};
        // Since C++17 `distance` can be used in constexpr context.
        static_assert(std::distance(il.begin(), il.end()) == 3);
        static_assert(std::distance(il.end(), il.begin()) == -3);
    }

Output:

    distance(first, last) = 3
    distance(last, first) = -3

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 940 | C++98 | the wording was unclear for the case where first is reachable from last | made clear

### See also

advance | advances an iterator by given distance (function template)
countcount_if | returns the number of elements satisfying specific criteria (function template)
ranges::distance (C++20) | returns the distance between an iterator and a sentinel, or between the beginning and end of a range (algorithm function object)

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